1. Field of the Invention
The invention concerns a diplex filter to filter signals and methods to filter signals by means of a diplex filter.
2. Description of the Prior Art and Background Information
An impedance-transforming diplex filter must necessarily exhibit bandpass characteristics. It is in fact theoretically possible to realize typical low-pass filters or high-pass filters that are dimensioned for identical impedances at the input and output such that such filters have the exact same transmission frequency response for different impedances. However, these typical low-pass filters or high-pass filters have a marked mismatching at these ports since the terminating impedance or the source impedance is not transformed. This is described by A. B. Williams, F. J. Taylor in “Electronic Filter Design Hand Book”, 3rd Edition, McGray Hill, 1995, for example.
Furthermore, the two diplex filter paths of a diplex filter must conventionally be high-resistance (high-ohmic) at their junction point or their connection node in the cut-off-state range, this exhibit a reflection factor close to +1. If a branch Y-A of the low-frequency branch and a branch Y-B of the high-frequency branch of the diplex filter are considered, a series inductor must be arranged first in the branch Y-A and a series capacitor must be arranged first in the branch Y-B. Furthermore, shunts cannot be present at the connection nodes to ground since these would represent a short in the other frequency range.
Diplex filters with bandpass characteristics thus can have, for example, series resonance circuits as a series element that are low-resistance at the frequencies of the branch Y-A and are high-resistance at the frequencies of the branch Y-B. In this regard, FIG. 1 shows a schematic circuit diagram of an example of a conventional impedance-transforming bandpass diplex filter half. FIG. 1 shows the branch Y-A with the series resonance circuit C2, L3 as a series element, the capacitor C1 as a shunt and a transformer Ü coupled between them with the coils L1 and L2 as well as the fixed coupling by means of a ferrite.
Furthermore, a tapped coil, two inductively coupled coils or a capacitive voltage splitter can also be inserted in the cross-branch at the ports A and B for impedance transformation.
For the last cited variant, FIG. 2 shows a schematic circuit diagram of a second example of a conventional impedance-transforming bandpass diplex filter half of the second order with the capacitive voltage splitter C1, C2. The variant according to FIG. 2, however, functions only within a relatively severely limited bandwidth.
The above, conventional, impedance-transforming diplex filters according to FIGS. 1 and 2 are known from G. Fritsche, “Entwurf passiver Analog-Vierpole” [“Design of passive analog quadrupoles”], Netzwerke 11, Page 206-214, Akademieverlag, 1979, for example.
However, the exemplary filter according to FIG. 1 has the disadvantage that special components such as transformers are required. Moreover, no ferrites are possible if the filter is to be used in a magnetic field (for example in a magnetic resonance tomography apparatus), which severely limits the realization of inductively coupled coils (as in FIG. 1) and causes greater losses.
Furthermore, impedance-transforming high-passes and low-passes of the fourth order are known from the publication “Transformierende Hoch- und Tiefpässe vierter Ordnung—exakter Algorithmus mit Beispielen” [“Transforming high- and low-passes of the fourth order—exact algorithm with examples”], Ulrich Fleischmann, Elektronikschau 6/1981, Pages 26-35. As already stated above, filters can be transformed due to their inherent properties across a finite bandwidth so that the terms “high-pass” and “low-pass” are meaningful only to a certain approximation. The low-pass thus also has a lower limit frequency as of which the transmission |S21| then decreases again. In an analogous manner, the high-pass has an upper limit frequency above which the transmission |S21| decreases, and in fact up to a minimum value:
                s      21            =                              1          -                                                  r                                      2                              ⁢                          ⁢      with      ⁢                          ⁢                      r                      =                                  Ϛ          -          1                          Ϛ          +          1                          
Naturally, it is possible in principle to interconnect the high-pass explained above and the low-pass explained above into a “diplexer” or into a “diplex filter” since these have a series element at the low-resistance port. However, the present inventor has established by means of tests that the cut-off effects fall off far too little in the respective unwanted frequency ranges (the cut-off frequency range), so that only an imperfect separation of the frequency ranges (transmission frequency range and cut-off frequency range) is possible. Furthermore, the present inventor has established by means of tests that, given an interconnection of both filters, these interact too strongly, such that the resulting frequency response can no longer represent a diplexer.
This problem is explained in detail using the example below and using FIGS. 3 through 7.
For this FIG. 3 shows a schematic circuit diagram of an exemplary embodiment of a conventional low-pass filter of the fourth order, and FIG. 4 shows a schematic circuit diagram of an exemplary embodiment of a conventional high-pass filter of the fourth order.
In this example, the transmission range D1 of the high-pass filter or high-pass branch according to FIG. 4 is 11 through 12.5 MHz and the transmission range D2 of the low-pass branch or low-pass according to FIG. 3 is 7.5 to 9 MHz. The respective terminating impedance Z1 is given 50Ω at the common port Y with the filters of FIGS. 3 and 4 interconnected. In this case, the impedance 4*50Ω=200Ω should appear at the ports A or, respectively, B in the respective transmission frequency range.
For ç=200/50, the following values for the first transmission frequency range D1 from 11 to 12.5 MHz of FIG. 4 result according to the teaching of the aforementioned publication “Transformierende Hoch- und Tiefpässe vierter Ordnung—exakter Algorithmus mit Beispielen”:L1=2.35 μH, C1=90.8 pF, L2=908 nH and C2=235 pFand the following values for the second transmission range D2 7.5 through 9 MHz result according to FIG. 3:C1=112 pF, L1=2.88 μH, C2=288 pF, L2=1.12 μH
To provide a diplex filter, the circuit diagrams of FIGS. 3 and 4 would have to be interconnected at the connection point or the node Y. For this FIG. 7 schematically shows the two transmission frequency responses of an interconnection of the low-pass filter according to FIG. 3 and the high-pass filter according to FIG. 4.
In FIG. 7 the reference character T1 thereby shows the transmission of the high-pass filter branch (or furthermore also of the first filter) given an interconnection of the high-pass filter according to FIG. 4 with the low-pass filter according to FIG. 3.
Analogously, in FIG. 7 the reference character T2 shows the transmission of the low-pass filter branch (or furthermore also of the second filter) given an interconnection of the high-pass filter according to FIG. 3 with the low-pass filter according to FIG. 4.
The reference character D1 also designates the transmission frequency range of the high-pass filter branch or of the first filter. The transmission frequency range of the first filter lies between 11 and 12.5 MHz, for example.
The reference character S1 also designates the cut-off frequency range of the high-pass filter branch. The cut-off frequency range S1 lies at 7.5 to 9 MHz, for example.
The reference character D2 correspondingly designates the transmission frequency range of the low-pass filter branch or second filter. The cut-off frequency range S1 lies between 7.5 and 9 MHz, for example.
Furthermore, the reference character D2 designates the transmission frequency range of the low-pass filter branch or second filter. The transmission frequency range D2 lies at 7.5 to 9 MHz, for example.
The reference character S2 analogously designates the cut-off frequency range of the low-pass filter branch or second filter. The cut-off frequency range S2 lies at 11 to 12.5 MHz, for example.
From the above values and FIG. 7 it is clear that D1 and S2 and D2 and S1 conventionally, respectively correspond to the design of a diplex filter.
However, FIG. 7 also shows that the interconnected filters, the low-pass filter according to FIG. 3 and the high-pass filter according to FIG. 4 interact, in particular in the range D2, S1 (7.5 through 9 MHz) and D1, S2 (11 through 12.5 MHz), such that they specifically do not reach the desired stringing together of the frequency responses of FIGS. 5 and 6, and therefore cannot form diplex filters.